1. Field of the Invention
The present invention relates generally to circuits for optical receivers, and more particularly to a level detector and automatic gain control for optical receivers.
2. Description of the Related Art
The delivery of video services over communication systems such as Hybrid-Fiber-Coax (HFC), Fiber-To-The-Curb (FTTC), and Fiber-To-The-Home (FTTH) often necessitates the use of high dynamic range technologies to support legacy analog NTSC signal formats. These video systems all use amplitude modulated (AM) optical carriers and require an optical transmitter to modulate the information onto the light. They also require an optical receiver to demodulate and amplify the signal for use by customer premise terminals, such as set top boxes or NTSC television sets.
A basic optical link used in an analog RF video delivery system is shown in FIG. 1. In this case, an FTTP analog RF system is shown, but the principles are equally applicable to HFC or FTTC systems. An optical transmitter takes in a multi-channel signal and amplitude modulates (AM) a light source in a linear fashion. The content can be standard NTSC analog TV channels or digitally modulated carriers such as used in cable modem systems. This system is primarily designed to carry video services, but quite often is used to carry advanced digital services such as high-speed data and telephony. The output of the optical transmitter provides an input to an Erbium Doped Fiber Amplifier (EDFA), which greatly increases optical power levels without adding significant noise or distortion. The EDFA high power output is then fanned out by means of an optical splitter to provide signal to a number of customers. Typically the fan-out or split ratio is 1:32 or 1:64. The amplitude-modulated optical signal is then demodulated into an electrical signal by a photo-detector, which functions as an envelope detector on the incoming light. A trans-impedance amplifier provides electrical gain such that the resulting signal is suitable for distribution to customer premise equipment, or to further coaxial distribution systems.
Because of the spatial diversity of customers and the variable nature of optical link budgets in typical deployments, optical path losses can widely vary. For instance, fiber runs will be longer in rural areas than in urban environments. Depending on the specific optical plant deployed, the number and locations of loss elements such as patch panels and splices will vary. To make wide-scale deployments over a large range of optical plants easier it is very desirable to have an optical receiver able to operate over a wide optical dynamic range. For instance, in some three-wavelength FTTP systems now in the early stages of deployment, the desired optical loss budget is between 10 to 28 dB. Unfortunately, optical receivers for 1550 nm wavelength video portions of the FTTP system only support about 7 or 8 dB of dynamic range. The small optical dynamic range of video optical receivers can make FTTP deployments more difficult since more effort must be expended to meet the relatively narrow optical input window. A wider 1550 nm wavelength video receiver dynamic range will make FTTP deployments easier.
To minimize cost of installation and maintenance, service providers often desire the RF output of their optical receivers to be held to a constant level over the optical dynamic range. As the input optical condition changes, it is desirable that the RF output level does not change such that the input level to RF equipment (such as television sets or set-top-boxes) is constant. To accomplish this, optical receivers often include capability for adjusting the RF output level using automatic gain control (AGC) circuitry.
A common technique for doing this is shown in FIG. 2. The output of the trans-impedance amplifier is sent to a variable attenuator whose loss characteristics are adjustable with a control signal. A variable gain circuit may also be used in place of the variable attenuator. The output of the variable attenuator feeds a direction coupler which couples a small portion of the signal leaving the receiver to a signal detection circuit. The output of the directional coupler then goes to the RF distribution network which often consists of coaxial cable, splitters, and terminal devices. The signal detector circuit senses how much energy is leaving the receiver and feeds a signal to an AGC circuit which contains a servo-mechanism for adjusting the variable attenuator. Should for any reason the output of the level detector be low relative to a reference inside the AGC control block, the AGC control block will reduce the amount of attenuation in the RF path, thereby increasing the RF output.
In this way, the AGC control block continuously adjusts the variable attenuator such that the RF output level is held constant over a wide range of anticipated variations. Known variations include changes in the depth of optical modulation index (OMI) on the incoming light, changes in input optical power level, variations in photo-detector responsivity, and changes in trans-impedance gain. These quantities can all vary with time, temperature, and from unit to unit, so the ability to automatically adjust receiver characteristics is very desirable.
It is important to note the directional nature of the coupler. Signal reflections from the RF distribution network should not be allowed to reach the signal detector, as would be the case if the directional coupler had poor ability to separate forward going signals from reverse ones. In the case of FTTP networks, the poor control over the impedance of RF distribution networks in homes requires good directional characteristics to insure proper output levels are launched from the receiver.
The problems with the AGC approach in FIG. 2 are cost and complexity. Broadband directional devices are often wound with multiple turns of fine wire around ferrite cores with small openings, which is a labor-intensive process. Eliminating the need for ferrite wound devices in the AGC circuit will lead to a direct cost reduction. Broadband couplers also introduce loss into the receiver output. Loss-loss directional couplers are possible but at the expense of the amount of coupling provided to the signal detector.
For best signal detector performance, it is desirable to have higher levels of signal incident on the detection device in the level detector block. Without adequate RF drive, level detectors will output correspondingly low levels of voltage that in turn makes it necessary to use expensive op-amps with higher levels of precision performance, such as offset voltage, in the AGC block. To alleviate this problem, RF post amplifiers are often used between the directional coupler's coupled port output and the signal detector input. This approach adds cost, however.
Optical receivers often perform output level control by sensing the input light condition, as done in Skrobko, U.S. Pat. No. 6,674,967, which is commonly known in the industry as “optical AGC”. Here, the amount of DC current drawn by the photo-detector is proportional to the amount of optical power hitting the receiver. This information may be used to adjust the output level of the receiver. When the input power is high, the receiver gain may be adjusted an amount that will hold the output level constant. When the amount of light is low, the receiver gain may be adjusted upward an appropriate amount to hold the output level constant. The drawback of this approach is it is an open loop implementation. There is no ability to adjust for variations in input light OMI or variations of the gain of the receiver.
Even in optical AGC approaches, it is often sometime desirable to have an indication of the amount of signal leaving the receiver. In those cases, a direction level detector is desirable even if the output of the level detector is not used in the adjustment of gain.
The frequency response of the trans-impedance amplifier (TIA) section of the receiver shown in FIG. 2 is in large part determined by the pole formed by the photo-detector junction capacitance appearing across the cathode and anode nodes, and the effective input impedance of the TIA. As noted in the prior art discussion of Cole, et al, U.S. Pat. No. 5,095,286, shunt feedback from output to input creates a trans-impedance amplifier and is effective in minimizing the effects of photo-detector capacitance and providing a wide bandwidth. As described in the discussion in Kruse, U.S. Pat. No. 4,998,012, proper selection of the value of shunt feedback inside the trans-impedance amplifier can result in satisfactory noise and distortion performance. In Cole, et al, U.S. Pat. No. 5,095,286, power gain of the receiver is determined by the ratio of input impedance to output impedance as seen by the amplifier, thus the frequency response and gain characteristics are largely determined by the large source impedances, on the order of 500 ohms, appearing at each photo-detector terminal. Trans-impedance amplifiers using shunt feedback will generally use values of feedback resistances of between 300 and 1000 ohms, which provides excellent low input impedance due to the negative feedback action, and excellent noise from the low amount of thermal noise from the resistors. Historically, these benefits have lead most designers to use shunt feedback in their trans-impedance amplifier designs.